Remove outdated `BigInt` support `FIXME` from `JSTypedArray`

[MaxDesiatov]

The linked issue has already been closed, so this comment doesn't seem to be relevant.

[github-actions[bot]]

Time Change: +571ms (3%)

Total Time: 17,767ms

Test name	Duration	Change
Object heap/Increment and decrement RC	5,357ms	+416ms (7%)	🔍

View Unchanged

| Test name | Duration | Change | | :--- | :---: | :---: | | Serialization/Write JavaScript number directly | 392ms | +15ms (3%) | | Serialization/Write JavaScript string directly | 428ms | +12ms (2%) | | Serialization/Swift Int to JavaScript | 5,742ms | +59ms (1%) | | Serialization/Swift String to JavaScript | 5,848ms | +68ms (1%) |